Trial spawners with pillagers always spawn them in blocks with block light level 9 or bellow. It is impossible to make trial spawners spawn pillagers in blocks with block light level 10 or higher, even with block_light_limit:[I;1,15] Moreover, it seems that block_light_limit imposes additional restrictions to allowed light level, instead of rewriting it. So trial spawner with block_light_limit:[I;10,15] will never spawn pillagers, at any light level.

Steps to reproduce:

1. Create a superflat world in creative
2. Execute the command to create spawner with pillagers with block_light_limit:[I;1,15] rule (You may need to run this and other commands in a command block, as it may be too long for the chat): /setblock ~ ~ ~1 trial_spawner{normal_config:{spawn_range:16,total_mobs:8,simultaneous_mobs:8,total_mobs_added_per_player:0,simultaneous_mobs_added_per_player:0,ticks_between_spawn:10,spawn_potentials:[{data:{custom_spawn_rules:{sky_light_limit: {min_inclusive:1,max_inclusive:15}

   ,block_light_limit:{min_inclusive:1,max_inclusive:15}},entity:{id:"minecraft:pillager"}},weight:1}]}}

1. Execute the next command to fill the entire area of spawn_range with light blocks with level 15: /fill ~18 ~ ~18 ~-18 ~ ~-18 light[level=15] replace air
2. Go to survival
3. Observe that no pillagers are spawned
4. Replace the pillager trial spawner with zombie or any other hostile mob trial spawner, leaving all his other settings untouched, by executing: /setblock ~ ~ ~1 trial_spawner{normal_config:{spawn_range:16,total_mobs:8,simultaneous_mobs:8,total_mobs_added_per_player:0,simultaneous_mobs_added_per_player:0,ticks_between_spawn:10,spawn_potentials:[{data:{custom_spawn_rules:{sky_light_limit: {min_inclusive:1,max_inclusive:15}

   ,block_light_limit:{min_inclusive:1,max_inclusive:15}},entity:{id:"minecraft:zombie"}},weight:1}]}} OR just by clicking on the spawner with zombie spawn_egg

1. Observe that mobs spawn (block_light_limit rule working)
2. Now you can also execute the /fill ~18 ~ ~18 ~-18 ~ ~-18 light[level=5] replace light[level=15] to replace the area with level=5 light blocks and execute the command from the step 2 one more time. As you may see, pillagers now start spawning.